Various methods exist to increase the mechanical strength of microporous separator membranes for lithium ion secondary batteries. One such method of improving the mechanical strength of a dry process microporous battery separator membrane is discussed in U.S. Pat. No. 6,602,593. This method is based on using a blow-up ratio (BUR) of at least 1.5 during blown film extrusion. As known to one skilled in the art, a blow-up ratio method involves a radial expansion of blown film from an annular die. An increased level of crystalline structure orientation in the transverse direction (TD) was achieved in the extruded membrane using a blow-up ratio equal to or greater than 1.5.
U.S. Pat. No. 8,795,565 describes a biaxial stretching technique involving both machine direction (MD) and TD stretching of a dry process precursor membrane with a controlled MD relax process step. Biaxial stretched membranes have improved mechanical strength in the machine direction (MD) and TD direction which may be predictive of excellent strength performance when used as a battery separator membrane in a lithium ion battery.
U.S. Pat. No. 8,486,556 discloses a multilayered battery separator with increased strength as defined by the Mixed Penetration strength test method which is a measure of the force required to create a short circuit through a separator membrane. A high molecular weight polypropylene resin with a melt flow index≤1.2 gram/10 minutes measured at the polypropylene layer in the PP/PE/PP trilayer configuration of the multilayer separator membrane was used to produce the multilayered separator with thicknesses ranging from 21 to 24.5 μm, a porosity ranging from 35% to 37%, an ASTM Gurley from 18 to 19 seconds (equivalent to a JIS Gurley=450 to 475 seconds), and Electrical Resistance (ER) (equivalent to the term Ionic Resistance, IR) ranging from 2.1 to 2.3 ohm-cm2.
Also known are wet process microporous battery separators which are also typically biaxially stretched and may have fairly balanced MD and TD strength properties. Examples of microporous membranes produced using a wet process may be U.S. Pat. Nos. 5,051,183; 6,096,213; 6,153,133; and 6,666,969.
Wet process battery separator membranes are manufactured using very high molecular weights polymer resins which typically have a molecular weights greater than 500,000 and more preferably are greater than 1,000,000 and require the use of a plasticizer(s) to allow melt extrusion. In addition, a component know as a plasticizer(s), typically oils, must be used in order for the very high molecular weight resins to undergo melt extrusion. The plasticizer must be extracted using solvents as part of the manufacturing process. The oil-plasticizer contaminated solvent from the extraction step of the manufacturing process must be recycled in order to bring the extracted solvent and oil to usable purity quality. This is an additional energy cost that is expensive. Therefore, the wet process has the disadvantage of being a possibly environmentally challenged process with costly solvent handling and disposal issues when compared to the solvent-free, ‘green’, low impact, less expensive, dry process method.
The known methods of BUR blown film method, TD stretching of dry process membranes and the wet process biaxial stretched porous membranes have yet to achieve excellent strength performance properties in combination with a low Electrical Resistance (ER), not only <2 ohm-cm2 ER range, but in the much lower and more preferred ER range of ≤1.3 ohm-cm2.
Accordingly, there is a need for a dry process, solvent free, environmentally low impact process that produces a microporous battery separator or membrane with excellent cycle performance and safety in a lithium ion battery. Battery manufacturers in the high power applications, for example, the Electric Drive Vehicle (EDV) industry, desire or require microporous battery separators with thicknesses preferably ranging from 14 to 30 μm, microporosity, with a high charge rate (C-rate) for optimal high energy performance. Furthermore, there is a need for a dry process microporous battery separator or membrane that will meet these requirements for EDV and Hybrid Electric Vehicle (HEV) battery systems.